Why can we never forget September 11th

At 8:45 am, September 11th, 2001, the first of two airlines struck the World Trade Centre in New York. At 9:03, a second plane crashed into a second World Trade Centre tower, and exploded. At 9:43 am, a third passenger plane crashed into the Pentagon, the world's largest office building. At 10:48, police confirmed that a fourth passenger jet had crashed in Western Pennsylvania.
September 11th marks the deadliest terrorist attack on American soil, and the last day of New York's World Trade Centre. Both the North and South Towers had collapsed, resulting in a massive cloud of debris, dust, and smoke. Everyone on the passenger planes that had been hijacked by terrorists was killed, as well as more than 3000 innocent people on the ground. Is it any wonder then that the day 11th September 2001 will always be remembered by us as one of the saddest days in modern times?